Using a hardware wallet – sometimes called “cold storage” – is widely accepted as the most secure method for storing cryptocurrency. It's backed by security experts and keeps your private keys offline – so your crypto is inaccessible to anyone but the holder of specific access codes.
As you may have discovered during your journey through investing in cryptocurrency, many or even most crypto-specific wallets don't work with multiple different cryptocurrencies. For example, Litecoin Core is a desktop app wallet that only stores Litecoin coins and tokens.